Electronic article surveillance systems have become commonplace in recent years as an effective tool for retail stores and libraries to protect against unpermitted removal of articles and books. For the identification of articles to be protected, these systems rely on a detection signal issued from a special marker affixed to said articles. There are several kinds of detection signals, and the selection of a suitable signal depends on the specific use. Methods of detection are roughly divided into the following three categories. The first approach utilizes a process which comprises magnetizing a special soft magnetic material. The second method makes use of an abrupt change in the impedance of an LC resonant circuit at a specified frequency. The third way concerns a signal transmission circuit that radiates special electric waves. Among these, the first method can supply markers at low cost and hence is predominantly used. While there are many versions of this method, they share a common feature in that the abrupt change which occurs in the magnetic properties of magnetic materials upon magnetization is detected in terms of a voltage induced in coils. Furthermore, the magnetic properties associated with the detection include magnetostrictive vibrations, high permeability characteristics and the squareness ratio of hysteresis characteristics.
In the early stage of their development, magnetic markers were of relatively large size in the form of ribbons or wires. However, recently, in order to increase the number of articles to which the markers can be affixed, namely to have the markers affixed to smaller articles, there has been a need to minimize the size of the markers. However, if an attempt at size reduction is simply applied to ribbons or wires, the effect of "demagnetizing field", or the tendency of a magnetic material to resist its own magnetization in the direction of an applied magnetic field, increases to thereby deteriorate the characteristics of the material as a magnetic marker. Hence, it has been difficult to reduce the size of markers in a ribbon or wire form.
Under these circumstances, thin films of various magnetic materials have been investigated in order to develop compact markers. For example, JP-A-4-232594 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") corresponding to U.S. Pat. No. 5,083,112 discloses a marker in the form of a multilayered thin film comprising a plurality of magnetic thin films which are interposed by nonmagnetic thin films. Each magnetic thin film is separated from an adjacent magnetic thin film by a nonmagnetic thin film. As a result, magnetostatic coupling develops between adjacent magnetic thin films to sufficiently reduce the demagnetizing field and allow for size reduction of the marker. However, in order to fabricate the marker, magnetic thin films must alternate with nonmagnetic thin films, thus resulting in a complex structure. In addition, the thickness of each nonmagnetic thin film must be controlled with sufficient precision to assure that adjacent magnetic thin films will be coupled magnetostatically. However, this has often caused fluctuations in the magnetic characteristics of the fabricated markers.
Unexamined published Japanese patent application No. Hei. 5-502962 which is based on a PCT application (corresponding to U.S. Pat. No. 5,455,563) discloses a magnetic marker having a thin magnetic film in which the surface is modulated to thereby improve its magnetic characteristics. According to this method, a sharp blade is applied to a thin amorphous metal film on a polymer substrate such that flaws are made at given spacings, to thereby magnetically partition the thin metal film and to provide a magnetic marker having satisfactory characteristics. However, it is difficult to manufacture magnetic markers of satisfactory characteristics in a consistent manner by processing the surface of thin films by either mechanical or chemical means. Moreover, the magnetic characteristics of the marker are potentially deteriorated rather than improved.
JP-A-4-218905 (corresponding to U.S. Pat. No. 5,181,020) discloses that a small thin-film magnetic marker having satisfactory characteristics can be produced by depositing particles on a substrate to form a thin film. The substrate is spatially positioned relative to a target such that the subject particles are incident at an angle with respect to the substrate normal. In practice, magnetic markers having satisfactory magnetic characteristics can be obtained when thin magnetic films are fabricated on organic polymer substrates by this method. However, the magnetic characteristics fluctuate with the type of substrate that is used.
In most all cases in the prior art, markers are successively affixed to articles by means of a dispensing machine as they are peeled from the surface of a roll 1 as shown in FIG. 2. Therein, the roll 1 has a plurality of magnetic markers 2 arranged longitudinally on a film 3 furnished with a release paper. However, to realize faster dispensing, there is a growing demand today for "transverse markers", which are peeled from the surface of a roll 1 as shown in FIG. 1. Roll 1 of FIG. 1 has a plurality of magnetic markers 2 arranged transversely on a film 3 furnished with a release paper. Although the need is ever increasing not only for transverse markers in a ribbon or wire shape but also for those in a thin film shape, few studies have been made to meet this need. Still less has been described in the three prior patents discussed above.
The present inventors previously found that when an organic polymer substrate in which the absolute value of the difference in the degree of thermal shrinkage between longitudinal and transverse directions ranged from 0.003 to 0.015 was used as a substrate for preparing magnetic thin films, satisfactory uniaxial magnetic anisotropy could be obtained. The present inventors filed JP-A-7-220971 which describes an invention based on that finding. Magnetic markers fabricated from such thin films display fairly good magnetic characteristics. However, there was still a need for further improvement.